What is the equation of the directrix of the parabola given by the equation (y − 3)2 = 8(x − 5)? a. y = 3 b. x = 3 c. x = 5 d. y = -5

Find the focus and directrix of the parabola with the given equation. Then graph the parabola. 
y^=-12x
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y^2=-12x
This is an equation of a parabola with a horizontal axis of symmetry. 
Its standard form: (y-k)^2=4p(x-h), with (h,k) being the (x,y) coordinates of the vertex.
For given equation:

Vertex(0,0) 
4p=12
p=3
Focus:(-3,0)
Directrix: x=3
see the graph below as a visual check on the answers:
..
y=±(-12x)^.5